Air Racing

paul330 · #1 02-13-2019, 04:04 AM

For the last 2 years I have taken part in Race for Rhinos in Botswana. Great fun but I want to move myself up the results table this year. It?s not Reno - it?s a handicapped navigation exercise over about 320nm on each of 2 days. Major mods to the aircraft are not permitted so I am trying to come up with a couple of ?tweaks? that might give me a knot or 2 - bearing in mind that an extra knot over the 2 days equates to about 1.5 minutes......

- ballast in the baggage hold. Reduces trim drag by shifting CG aft. However, ups the gross weight.

- rig ailerons (say) 1 degree up. Less lift produced in the outboard wing section and more inboard so reduction in tip vortex drag.

- I have Slick mags with a fixed 25 deg advance. Retime to (say) 27. Possible downside - increased CHT and possible starting problems.

Comments/analysis from the team welcome ......

Tom Martin · #2 02-13-2019, 05:06 AM

Remove any external Nav or second com antennas that are not critical to the flight, this should be good for at least a knot.
Take a good look at your gear farings and wheel pants. We know that you get 12 to 15 knots by using the farings so it only makes sense that a bit of tape might also pay with some speed
And most importantly, get a good weather briefing regarding upper winds. Practice some climbs and find out how much of a tail wind is worth climbing for. The reverse also applies to headwinds. Keep in mind that moderate turbulence can cost 2 to 3 knots.

paul330 · #3 02-13-2019, 05:20 AM

Removal of equipment and taping not allowed under the rules.

Yes, of course, winds are taken into account. Worked that out as a break even of about 5kts per 1000'. Some seriously low flying last year on the into-wind legs - this is Africa after all ....

On the other hand, you have to take into account ease of navigation. This is map, compass and stopwatch with GPS disabled so low flying to gain a couple of knots can be counterproductive if you wander off track.

paul330 · #4 02-13-2019, 05:38 AM

On the other hand, if one were to remove the extra aerials, amend the equipment list, collar CBs and label the COMM "ILS/VOR Inop" before submitting the entry...... Hmmmmm......

Not entirely in the spirit of the rules but

However, analysis of my other potential tweaks still welcome....

MConner · #5 02-13-2019, 06:53 AM

I have heard that racers clean and wax their prop daily.

Alan Carroll · #6 02-13-2019, 06:59 AM

Push the throttle and prop controls all the way forward! This might seem obvious, but based on Airventure Cup results over the years I suspect there were a few pilots who were not really racing. Along the same lines, lean for best power (and be prepared for some really impressive fuel flow numbers!).

Ideally you should test the effects of any changes, but it's extremely difficult to accurately and reproducibly measure changes of 1-2 knots. It might be worthwhile to do some climb tests however, and to test true airspeed at different altitudes (see the archives for how to do this). Keep in mind that the best race altitude depends both on the wind, and on how long you'll be there.

paul330 · #7 02-13-2019, 07:04 AM

Thanks guys. Without wishing to sound ungrateful for the comments, I?ve learnt all this stuff over the last couple of years. I?m looking for some sort of guidance as to whether my less obvious ideas above are likely to be practical or will actually achieve anything.

Toobuilder · #8 02-13-2019, 08:58 AM

Not sure what altitude you are going to fly but I can tell you that the timing bump you have proposed will probably not have a measurable effect on speed. I did a timing sweep at best power mixture and 8,500 feet and found the power essentially flat. My 540 D4A5 did peak in speed at 27 degrees, but that was only worth one knot over the standard data plate setting of 25 degrees. If you fly lower, this slight "peak" will diminish even further. Of course, the inverse is true as well. If you do bump the timing hoping for a slight edge up high, you will be too advanced while climbing to that altitude so the net result might be zero gain.

paul330 · #9 02-13-2019, 08:35 PM

Thanks Michael.

Actually, the base is at 3000' and we don't really climb much. Last year, I didn't get above about 1500' agl catching tailwinds on the down-wind legs. (you can probably add about 1000' for density altitude).

One knot is good! Any issues with starting with the retard breaker firing 2deg earlier?

Toobuilder · #10 02-13-2019, 09:10 PM

I'll revise my statement. At your altitude, you will not see any gain in speed due to an ignition advance. I have done full throttle, sea level testing and I've swept from 20 to 30 degrees and found ZERO speed difference. Said another way, with the rich mixture and fat air you will see, even a 5 degree retard from data plate values does not hurt speed.